Recently, electrophotographic image forming apparatus such as printers, facsimile machines or multifunctional peripheral (MFP) improved in energy saving are required for global environmental protection, and development of MFP reduced in electric cost has been progressed.
It is essential for reducing electric cost to attain low temperature fixing in the fixing apparatus consuming large electric power. Also, airtightness of the image forming apparatus and the toner cartridge has to be improved since small sizing of the apparatus and maintaining suitability are necessary. The toner to be used in such the apparatus is desired that the toner particles are not thermally coagulated by accumulating caused by the fixing treatment. Concretely, it is essential that the toner particles are not thermally coagulated and have stable fluidity and superior heat resistive storing ability in a magnetic single-component or non-magnetic single-component toner cartridge or a double-component developer type developing unit in which a developing roll and a magnetic carrier are enclosed.
On the other hand, the electrophotographic image forming method is spread from simple use for copying documents in an office by a usual copying machine to a printer to uses out of the office. Specifically, application of the electrophotographic image forming system expands to the market of on-demand printing (POD) included in the area of light printing. On the POD market, need of printing on recording materials other than usually used image recording material such as paper in the electrophotographic system become strong.
The paper suitable for the electrophotographic image formation is generally designed to be suitable for the image forming apparatus, namely suitable for the toner used in the apparatus, where the surface resistivity of the paper is increased to increase the transferring efficiency, or a filler having small particle diameter or having low damage causing property is applied to reduce the damage on the photoreceptor. However, it is required in the field of POD that the tone can be suitable for various kinds of paper.
Concrete examples of the various kinds of paper include a cardboard to be used for a cover of booklet, an advertisement poster, a glossy paper to be used for a printed matter having high quality feeling and a coated paper to be used as a stout card on which wax or polylactic acid latex was coated. Moreover, in the field of POD, there is a case in which a coated paper is used as the printing paper and then laminate treated after printing to increase the glossiness of the whole of the printed matter.
Various problems occurs, however, on the occasion of printing using the usual toner onto the above described kinds of paper. For example, a problem such as that the toner is peeled off from the coated paper because the toner is not sufficiently permeated into the coated paper so that the fixing is insufficiently performed when the usual toner is printed and fixed on the coated paper, and that the toner is peeled off from the laminating material when the paper is subjected to the lamination treatment because sufficient contact between the toner and the laminating material cannot be obtained. Therefore, sufficiently fixed visible image is difficultly to be formed when the printing is performed onto the above-described kinds of paper by using the usual toner.
It has been known to use a graft-copolymer or a block-copolymer of crystalline polyester and amorphous polyester as a binder resin constituting the toner for attaining low temperature fixing in the fixing apparatus. It has been found that the low temperature fixing ability can be obtained by high melting ability of the crystalline polyester and by high viscoelasticity of the amorphous polyester when an image is printed using such the toner on the above-described paper such as the coated paper.
However, the binder resin formed by simply graft-copolymerizing or block-copolymerizing the crystalline polyester and the amorphous polyester results in having a usual polyester structure and is negatively charged. Therefore, there have been problems, for example, that, when such a binder resin is used in a toner, the amount of charge is unintentionally increased, or that only insufficient adhesion of the toner to the coated paper is obtained due to repulsion force of the charge when printed on a paper coated with polylactic acid. As the result, the toner tends to peel off at the interface of the coated paper and the toner with a strong impact, when the image printed on the coated paper is subjected to a laminating treatment. It can be considered as the reason of the peeling off of the toner from the polylactic acid coated paper that, not only due to the electrostatic repulsion force between the coating material and the toner, sufficient adhesion cannot be obtained when the toner does not have a high melting performance in the case of the fixing of the toner by thermally melting the binder resin, because the coating material of the coated paper is not melted to deform as a usual paper sheet so that the anchor effect cannot be obtained. In the case of the ink, suitable fixation can be obtained because the ink is permeable into the coated paper.
As above-described, the toner using the binder resin formed by simply graft-copolymerizing or block-copolymerizing the crystalline polyester and the amorphous polyester does not exhibit sufficient toner strength even though sufficient low temperature fixing ability can be obtained.
The crystalline polyester easily causes filming on triboelectricity generating parts such as the developing roller, the carrier and the image carrier when the binder resin composed of the crystalline polyester becomes exposed from the toner particle by crashing the toner particle since the crystalline polyester has malleability. When the filming is caused on the image carrier, a problem is posed that the filming resin absorbs moisture and causes irregular image such as image streaming at the starting of the image forming apparatus.
For solving the problem of filming, a technology is proposed, in which the crystalline polyester is graft-copolymerized as a branched chain with a principal chain of another resin, cf. Patent Documents 1 to 3, for example. However, the problem of filming is not sufficiently solved yet by such the technology.
Reason of that is surmised that the resin is not sufficiently grafted because the graft-copolymerization is performed by applying the dehydration reaction on the occasion of synthesizing the polyester.
Patent Document 1: Japanese Patent Application Publication Open to Public Inspection (hereafter referred to as JP-A) No. 5-45929
Patent Document 2: JP-A No. 5-44027
Patent Document 3: JP-A No. 2006-18018